The present invention relates to a process for producing a carboxypolysaccharide, and more particularly to a process for producing a carboxypolysaccharide or a salt thereof by oxidizing a polysaccharide in the presence of a ruthenium compound and ail oxidizing agent while recovering ruthenium to reuse it. The carboxypolysaccharide and the salt thereof obtained by the process of the present invention is advantageously used as a scale deposit inhibitor, a pigment dispersant, a sizing agent, a mixing aid for concrete and a builder for detergents.
As a builder for detergents, a polymer of acrylic acid or a copolymer of acrylic acid and maleic acid have heretofore been produced industrially. However, these synthetic polycarboxylic acids are very difficult to be biodegraded by microorganisms due to its vinyl polymer structure which is a chemical structure not generally found in natural macromolecular substances. Therefore, a polycarboxylic acid produced by carboxylation of a polysaccharide is expected to be useful as a biodegradable builder, solving the above problem.
For example, a process for oxidizing a polysaccharide with a salt of hypochlorous acid or a combination of periodic acid and a salt of chlorous acid is disclosed in Japanese Patent. Publication No. 49-1281. It is described therein that a dicarboxystarch obtained by oxidation of C2 and C3 positions of a monosaccharide unit constituting the polysaccharide is usable as a builder for detergent. Japanese Patent Publication No. 47-40552 and Czechoslovakian Patent No. 235576 disclose a method for producing a tricarboxypolysaccharide by converting a starch to an aldehyde with a salt of periodic acid and then carboxylating the aldehyde by oxidation with dinitrogen tetraoxide. However, since an expensive salt of periodic acid must be used in the proposed processes, the industrial application of these processes is costly disadvantageous.
Japanese Patent Application Laid-Open No. 9-71601 discloses a process in which a starch is carboxylated with a salt of hypochlorous acid in the presence of a ruthenium catalyst while adjusting pH of the reaction system to a basic condition. Since ruthenium used in this process is a very expensive transition metal, it has been strongly desired to minimize the loss of ruthenium during the production of a tricarboxystarch as well as to recover and reuse ruthenium. However, although Japanese Patent Application Laid-Open No. 9-71601 teaches the improvement in color of the polycarboxylic acid by treating the oxidation product with a reducing agent such as a salt of sulfurous acid and filtering off the resulting precipitate, the document considers nothing about decreasing the content of ruthenium in the final polycarboxylic acid as well as recovering and reusing the ruthenium catalyst.
Japanese Patent Publication No. 6-31840 discloses a process for removing radioactive ruthenium from a solution of a used nuclear fuel. However, the recovery and reuse of ruthenium in the process of producing a polycarboxylic acid (carboxypolysaccharide) are not taught therein.
Thus, no industrial process for recovering and reusing ruthenium in a process for producing a polycarboxylic acid (carboxypolysaccharide) has been proposed in the art.